In order to provide for communications between a fixed point, for example a geostationary satellite, and a moving point, for example a vehicle on the ground, an antenna making it possible to hunt down the fixed point is disposed at the level of the moving object. The constraints to be adhered to by this antenna are severe. Notably, it must be configured so as not to emit in other directions signals with a power density greater than a regulated level, so as not to disturb the service provided for by adjacent satellites. A relatively high precision in the tracking of the satellite must therefore be guaranteed with this type of antenna. By way of example, for coverage of the European continent, the reflector of an antenna on the ground (or on an airborne carrier) must be able to be oriented in relation to an interval of angles lying between about 10° in elevation for Spain and 60° for northern Europe, the reflector being 360° orientable in relation to the azimuth angle. The reflector, with a diameter of about 60 to 70 cm, must thus benefit from a considerable freedom of movements and from a reliable and precise control system, thus leading to bulky and expensive antennas. Moreover, when the polarization of the signals is linear—if for example the satellite comprises an antenna with a single source of signals—, the ground antenna must be constantly aligned with the direction of polarization.
In order to lessen the constraints to be satisfied by ground antennas and thus simplify their production, circular polarization may be employed in place of the aforementioned linear polarization, for example in the Ka band. By way of illustration, the frequency band lying between 19.7 GHz and 20.2 GHz can serve in reception at the satellite level, while the band lying between 29.5 GHz and 30 GHz may be used in emission, coverage being provided for by a set of adjacent spots in right or left circular circulation.
Multibeam satellites cover a territory with a plurality of spots configured in such a way that the signals emitted on two neighbouring spots do not interfere. In addition, the coverage of a satellite comprises spots having various transmission frequencies and/or various polarizations, two neighbouring spots being configured so as not to have, at one and the same time, the same polarization and the same transmission frequency. The frequency characteristics and polarization characteristics of the signals emitted on a spot are generally designated by the expression “spot colour”, two neighbouring spots therefore having distinct colours. By way of illustration, with two different polarizations and two different transmission frequencies, four colours of spots may be created.
Antennas onboard mobile craft required to provide for communication with a satellite sometimes cross a boundary between two spots. This is the case, for example, with antennas intended to provide an Internet connection from an aircraft or a train. When the antenna leaves the zone covered by a first spot configured with a first polarization (for example right circular) and enters the zone covered by a second spot configured with a second polarization (left circular), the antenna must switch rapidly so as to modify its emission and/or reception polarization. Furthermore, the radiating elements of a beamforming antenna must be sufficiently close together to avoid the formation of lateral radiation lobes, liable to perturb adjacent communication systems.
A publication by Kwang-Seop Son et al., published in 2006 in “Proceedings of Asia-Pacific Microwave conference” under the title “Waveguide Slot Array In-Motion Antenna for Receiving both RHCP and LHCP using Single Layer Polarizer”, discloses an antenna structure comprising sources of signals exciting polarizers aligned on a film. The polarizers are arranged alternately in opposite directions and the sources are separated from the film of polarizers by a radiofrequency-insulating layer and provided with a series of cavities placed facing the polarizers in such a way that at a given instant, one polarizer out of two is illuminated by a source. The film may be actuated in translation so that the cavities are placed facing the polarizers which were not previously illuminated. These polarizers being oriented in a different direction from the first polarizers, the polarization of the signals emitted by the antenna is reversed. This antenna therefore makes it possible to carry out a switching between two different polarizations. However, it comprises drawbacks. Indeed, its structure imposes a relatively large distance between the radiating elements, thereby giving rise to overly sizable lateral lobes in the radiation pattern.